c. T-4 theodolite 



This is the largest and most accurate of the Wild theodolites. 

 It is built primarily for astronomical work and is capable of precise 

 azimuth determination. The T-4 is a broken-telescope type of transit 

 in which the eyepiece is at the side and always horizontal so that the 

 tilt of the telescope in no way impedes the sighting. It can be used for 

 all well known methods of astronomical determination of position. Such 

 use involves other units, however, and is not accomplished with the T-4 

 alone. Astro observations are taken with the T-4, chronometer, and 

 radio receiver, all connected to a chronograph which records on tape 

 the data from all three sources. The theodolite marks the time of 

 transit of a star; the chronograph provides local sidereal time; and 

 the radio supplies time signals from the National Bureau of Standards. 

 Breaks in the electrical contact are made by time ticks and transit 

 marks and show on the tape in their proper order. This system gives 

 a reliable radio- chronometric comparison to provide the necessary 

 accuracy of position. 



The total weight of the T-4 and accessories is about 400 pounds, 

 depending upon the size of the radio receiving set and generator. The 

 T-4, itself, is shipped in two boxes: in one, the telescope weighing 88 

 pounds, and in the other, the base weighing 121 pounds. Direct readings 

 can be made on the horizontal circle to 0.1 second and on the vertical 

 circle to 0.2 second. With a sensitive suspensionlevel and two Horrebow 

 levels (all with scales of one second for each two millimeters), first- 

 order astronomical positions can be obtained. The T-4 has worked 

 well even under adverse conditions; in remote areas, difficulty in the 

 reception of time signals becomes the most troublesome factor. 

 Improvement in radio reception might be obtained through use of a 

 more powerful receiver and an improved antenna. The cost of a com- 

 plete T-4 theodolite, with accessories, is about $9,000. 



3. Levels 



Vertical control for topography and to connect tide gauges to 

 bench marks while important, is a small portion of the total field work. 

 It is sufficient to say that vertical control can be established when 

 needed. Wild levels, Models N-II and N-III, generally are used and, 

 like the theodolites, are considered compact, sturdy, and accurate. 

 First-, second-, and third-order control are possible with them. 



XV- 3 



